It has long been recognized that the electrical machining of a conductive workpiece, e.g. the electrical-discharge machining, electrochemical machining or electroplating of workpiece portions juxtaposed with an electrode, is often characterized by a non-uniform current distribution across the gap separating the electrode surface from the workpiece surface to be machined. This nonhomogeneous current distribution mostly derives from a contamination of the machining medium in the form of accumulations or concentrations of ions, machining chips and other gap products, along one or the other surface of the electrodes. Moreover, the non-uniform distribution of the flow of current between the surfaces was also found to be, in part, a function of magnetic effects resulting from the passage of current between the electrode and the workpiece.
In U.S. Pat. No. 3,252,881 issued May 24, 1966 to Kiyoshi Inoue, it was pointed out that it was possible to effect mechanical dislodgment of ionic contaminants in an electrochemical machining gap by applying to the electrode a mechanical oscillation toward and away from the workpiece at a relative low or sonic frequency (e.g. from 10 cycles/second to 10 kilocycles/second). It was also shown that a similar result was obtained when, concurrently with the mechanical vibration of the electrode, the injection of a gaseous fluid into the electrolyte or as an alternative thereof, a supersonic vibration is applied to the electrolyte within the electrode. The supersonic vibration can have a frequency ranging between substantially 10 kilocycles/seconds and 10 megacycles/second and can be produced by an electrosonic transducer mounted within the interior of the tubular electrode.
It has been found that the ultrasonic vibration may be employed in electrical-discharge machining and electroplating as well to remove the gap contaminants in these processes. For example, in electrical-discharge machining, it has been found that the ultrasonic vibration serves to stabilize the machining condition and protect the workpiece and the electrode from short-circuiting damage. In electroplating, the accumulation of electrolytic bubbles tends to be removed as a result of imparting an ultrasonic vibration to the electrolyte so that a fine plated layer may be obtained on the workpiece surface. In these processes it has been the conventional practice to impart an ultrasonic vibration to the machining liquid by means of an ultrasonic transducer element which is simply immersed therein in a worktank or attached to the wall of the latter.
The transducer element immersed in the machining liquid in the worktank has commonly the individual vibrating surfaces on its opposite sides in contact with the machining liquid. It has now been observed that the ultrasonic vibrations individually emitting from the two surfaces tend to interfere with each other in the machining liquid so that only the composite vibration which is much damped in amplitude and energy is available to the region of the machining gap. The compensation for the large loss of energy requires an increase in the input power which results in an excessive heating of the transducer element and the consequential damage thereof or detrimental effects to the machining liquid.
Furthermore, when the shape of a workpiece contains irregular curvatures or involves a deep boring or slitting and thus represents two-dimensional or three-dimensional forming, it has been found that a uniformity of the gap decontamination effect is not attainable by means of a transducer element if arranged in one or another manner as done heretofore. It has thus been a problem with conventional ultrasonic vibrator systems to effect the gap decontamination satisfactorily, efficiently and uniformly over the entire working area being processed by electrical machining.